1. Field of the Invention
The present invention relates to electromagnetic fluid flowmeters for use on vessels of the type which do not project into the stream of fluid flow, and flowmeters for use in measuring oceanographic currents and water movement in other aqueous bodies. Such devices are used as speed indicators for nautical vessels, ocean current measurement devices, and velocity measurement devices for fluid flow in conduits.
2. Description of the Prior Art
In the past, a number of different types of flowmeters have been employed which operate upon the principle of sensing a voltage between two electrodes in contact with a flowing fluid. A voltage field is induced in the fluid by a magnetic field set up by electromagnetic conductive windings. This field, in turn, impresses a voltage differential between the signal electrodes proportional to the velocity of fluid flow. In such devices the sensing or signal electrodes are spaced laterally from each other along a line perpendicular to the direction of fluid flow. An elongated wound electrical coil is typically used to set up an electromagnetic field which field extends at least partially into the flowing fluid.
The induction coil configuration conventionally used to produce the electromagnetic field is cylindrically wound and arranged generally perpendicular to fluid flow with an extremity extending substantially to the plane of a solid boundary across which the fluid flows. The result is a magnetic field which lies partially within the flowing fluid, and the flux lines or vectors of which approach a perpendicular orientation relative to the boundary at the interface of the flowing fluid with the alignment of the induction coil core. As a result, an electrical potential is generated between electrodes spaced on either side of the inductor and in contact with the fluid. The vector of this electrical potential is perpendicular to the flux lines of the magnetic field at the fluid interface with the boundary surface. The electrical potential between the spaced electrodes is porportional to fluid velocity in the direction perpendicular to the orientation of the electrodes and also perpendicular to the lines of magnetic flux set up by the inductor at the fluid boundary interface.
One conventional device which operates by sensing a voltage differential set up in a fluid between sensing electrodes is known as a "rod meter" and is described in a number of publications, including U.S. Pat. Nos. 2,969,673 and 3,677,082. In devices of this type, the electromagnetic field employed is generated by an electromagnet located in a rod extending perpendicular from a solid surface across which a fluid flows. Sensing electrodes are mounted on the sides of the rod and are in contact with the water flowing past it. While such a device functions adequately, the rod itself induces a certain amount of turbulence and/or drag relative to the flowing fluid. Both turbulence and drag are particularly disadvantageous when flowmeters of this type are used as speed indicators in nautical vessels, and especially small sailing vessels.
Other flowmeters which sense electrical potential in a field which is altered depending upon the velocity of fluid flow past spaced electrodes include devices which protrude only slightly into the flowing fluid. Such instruments exhibit very little drag, when mounted on a nautical vessel and induce only negligible turbulence in fluid flowing through a conduit. However, all such conventional devices have required an elaborate mounting system relative to the surface across which the fluid flows. This has been necessary because of the physical configuration of all conventional devices of this type, which has required the structure of the flowmeter to extend interiorally a considerable distance from the mounting bulkhead or conduit wall opposite the surface in contact with the fluid. As a result, sophisticated and expensive through-the-hull fittings are required in order to accomodate conventional flowmeters. Moreover, the protrusion on the inner surface of the hull is both space-consuming and renders the instrument vulnerable to damage from any movement occurring in the vessel within its immediate vicinity. Likewise, the protrusion of conventional flowmeters of this type from the exterior of a fluid conduit renders protection from the elements quite difficult.
An additional disadvantage of conventional flowmeter systems is that the orientation of the induction coil and the sensing electrode leads must be exactly parallel to each other and perpendicular to the direction of fluid flow opposite the surface of fluid contact. Thus, even minor movement or contact with the exposed components results in serious inaccuracies in the measurement of fluid velocity.